1. Field of the Invention
The present invention is a device for mixing air and abrasive for use in artistically etching glass by abrasive blasting.
2. Description of the Related Art
Various means are employed to etch designs into glass. One means is to employ a chemical etching agent, such as hydrofluoric acid on the portion of glass to be etched. Chemical etching agents do not provide adequate control over the degree of etching achieved in order to be practical for artistic etching. Also, use of hydrofluoric acid presents potential health and safety concerns as it will aggressively attach human tissues if the user accidentally gets the hydrofluoric acid on his body.
Another means which is widely used to etch glass is a grinding wheel, usually a copper wheel, which can-be used to cut the glass. This means is generally employed to produce fine cut glass and lead crystal pieces.
A means which has more recently appeared for etching glass is to use various types of heated materials which are applied to the glass in order to chip the glass thermally.
A final means which is used to etch glass is by abrasive blasting. Various mixing devices have been employed to mix a pressurized carrier, such as air, with an abrasive, such as sand, so that the mixture of air and abrasive can be delivered where it is needed in order to etch the glass. In order to do artistic abrasive etching, it is critical to be able to precisely control the ratio of air to abrasive flowing from the delivery gun during the entire time the delivery gun is activated. Also, the flows of abrasive and air must be adjusted in order to achieve varying degrees of etching depth. Specifically, the abrasive rate of flow has to be simultaneously varied from a low rate of about 2 ounces per minute for shadow etching up to a maximum rate of about 14 pounds per minute for deep etching, and the air pressure has to be varied from a low pressure of about 3 pounds per square inch (psi) up to a maximum pressure of about 50 psi.
Controlling the differential pressure across the mixing valve is also critical so that pressure in the mixing chamber is always less than or equal to, but is never greater than, the pressure on the abrasive contained in the vessel. If the differential pressure is not properly maintained, flow of abrasive ceases because flow through the mixing valve is reversed, sending air from the mixing chamber, through the mixing valve and into the vessel, instead of allowing the normal flow to occur, i.e., flow of abrasive from the vessel, through the mixing valve and into the mixing chamber.
The present invention is capable of producing consistently correct air to abrasive ratios in its air/abrasive mixture over a wide range of abrasive flow rates and air pressures while constantly maintaining proper differential pressures across the mixing valve. The present invention controls the ratio of air to abrasive flow by three means. First, the ratio is controlled by controlling air flow to a head space of the vessel by means of an adjustable air regulator. Second, the ratio is controlled by controlling the abrasive flow from the vessel to the mixing chamber. The abrasive flow is controlled by means of an adjustable mixing valve and by means of the differential pressure created between the mixing chamber and the vessel which are connected via an air delivery line extending from the vessel's head space to the mixing chamber. The air delivery line supplies air to the mixing chamber and insures that the differential pressure between the vessel's head space and the mixing valve is either positive, i.e., the head space's air pressure exceeds the mixing chamber's air pressure, as when abrasive blasting is being performed, or zero, i.e., the head space's air pressure is the same as the mixing chamber's air pressure, as when abrasive blasting has ceased. And third, the ratio is controlled by limiting the amount of abrasive which accumulates in the mixing chamber when abrasive blasting is temporarily suspended, thus preventing an increased proportion of abrasive from being included in the air/abrasive mixture when abrasive blasting resumes.
Important objectives of the present invention include the provision of a mixing device capable of having flows of both air and abrasive easily adjusted while the device is in service, a mixing device which prevents significant buildup of abrasive in its mixing chamber when abrasive blasting is temporarily suspended, a mixing device which will consistently deliver a given air to abrasive ratio, a mixing device which constantly maintains a desirable differential pressure across its mixing value and a mixing device with a straight mixing chamber in order to reduce abrasion to the device by the air/abrasive mixture.